The subject invention relates to chemical blowing agents which also act as curatives for ethylene/carbon monoxide copolymers in the formation of closed cell microcellular articles.
It is well known in the art that heating a composition containing an ethylene based copolymer, a chemical crosslinking agent and a chemical blowing agent in a pressurized mold will crosslink the copolymer and decompose the blowing agent so that on opening the mold the composition expands into a closed cell microcellular foam. Articles such as flat shoe soles, foamed matting and recreation vehicle fenders are manufactured in this way.
During the molding process, the composition reaches a suitable degree of cure before the pressure is released and the mold opened. If the mold is opened before the necessary degree of cure is reached, the polymer network is not strong enough to contain the gaseous blowing agent and the article collapses as the gas escapes from the molten mass.
Combination chemical blowing and curing agents have heretofore also been disclosed in the art. For example, U.S. Pat. No. 3,428,583 discloses use of polyazo compounds to both cure and blow polyolefin resins; U.S. Pat. No. 3,846,350 discloses use of azodicarbonamide as a combination blowing agent and cure accelerator for polychloroprene rubber; U.S. Pat. No. 3,282,864 discloses use of cycloalkane di(sulfonyl azide) compounds to both cure and foam hydrocarbon polymers such as polyethylene, polypropylene and blends thereof; and Great Britain No. 2,009,183A discloses certain azo esters which can function as both blowing and crosslinking agents for polyvinyl chloride, ethylene copolymers, diene rubbers and poly(vinyl chloride) at temperatures greater than about 170.degree. C.